1. Field of the Invention
The present invention relates to a signal processing circuit for video signals each composed of chrominance signals different in signal level from one another, and more particularly, to a signal processing circuit used in an apparatus, such as an electronic endoscope, in which color sequential signals are processed.
2. Description of the Prior Art
In the electronic endoscope, illuminating light rays in red (R), green (G) and blue (B) are projected, as sequentially selected, to an object, video signals resulted from the illuminating light rays are provided as outputs sequentially as the time passes (color sequential signals), stored once into a memory means separately for respective chrominance signals and then read out from the memory simultaneously for reproduction as a composite video signal.
In such electronic endoscope, a solid-state image sensor (such as CCD, etc.) is usually used as the image sensor.
Since the R, G, and B chrominance singals are different in signal level from one another because of the frequency response characteristics of the solid-state image sensor, the output video signal from the solid-state image sensor will be color sequental signals different in signal level for R, G, and B signals, respectively, as shown in FIG. 2(1).
FIG. 2(1) shows each of chrominance signals in every vertical scanning period (1V). Color sequential signals in this period (1V) are shown in detail for every horizontal scanning period (1H) in FIG. 2(2). As seen from FIG. 2(2), a noise (level V.sub.d is caused by a dark current through the solid-state image sensor or other noises during the blanking period in the period (1H).
In the prior-art signal processing circuit, the gain of the amplification circuit which amplifies the signal from the solid-state image sensor is changed timely for the R, G and B chrominance signals, respectively, and these signals are so amplified as to have a same signal level as shown in FIG. 2(3).
As mentioned above, since different gains of amplification are assigned to chrominance signals, respectively, in the amplification circuit of the prior-art signal processing circuit, the amplifier gains for the G and B signals of lower signal levels are large as compared with that for the R signal, so that the noise level in the horizontal blanking periods of the G and B signals are also higher than that of the R signal. Assume that the noise level of the R signal is V.sub.d as shown in FIG. 2(3). The noise level of the G signal is A.sub.1 V.sub.d and that of the B signal is A.sub.2 V.sub.d.
Note that A.sub.1 and A.sub.2 are ratios between the gains of the amplification circuit for amplification of the G and B signals, respectively, and the gain for the R signal.
Therefore, when the black level of output signal of such amplification circuit is clamped (namely, an absolute voltage value of a video signal at the black level is set), the mean value of the noise levels V.sub.d A.sub.1 V.sub.d and A.sub.2 V.sub.d of the chrominance signals, respectively, is clamped as an apparent black level as shown in FIG. 2(4).
As a result, the true black levels L.sub.1, L.sub.2 and L.sub.3 are such that the potential of the R signal at the true black level L.sub.1 is the highest, that of the G signal at the true black level L.sub.2 is the second highest and that of the B signal at the true black level L.sub.3 is the lowest as shown in FIG. 2(5).
Thus, when chrominance signals different in true black level from one another are simultaneously read out of the aforementioned memory means and processed into a composite video signal, a color imbalance of reproduced image will result and the color reproducibility be degraded.
To eliminate the above disadvantage of the prior-art signal processing technique, it was proposed that DC voltages of different values should be added to the output signals, respectively, from the clamp circuits synchronously with the R, G and B signals, respectively, so that these chrominance signals have a same black level.
However, in case of the above-mentioned addition, synchronous with the R, G and B signals of DC voltages of different values to the chrominance signals, it is difficult to adjust such DC voltage value settings and the color reproducibility of a reproduced image is poor due to even a slight deviation of the set values. Also each time the electronic endoscope is used with the existing image sensor replaced with another image sensor having an image sensor different characteristics from those of the image sensor in the existing electronic endoscope, it is necessary to set new DC voltage values for the electronic endoscope going to be used. Namely, the prior-art signal processing circuit in the electronic endoscope has only a low compatibility and is extremely poor in operability.